1. Field of the Invention
In at least one embodiment, the present invention is related to catalyst supports for fuel cells.
2. Background Art
Fuel cells are used as an electrical power source in many applications. In particular, fuel cells are proposed for use in automobiles to replace internal combustion engines. A commonly used fuel cell design uses a solid polymer electrolyte (“SPE”) membrane or proton exchange membrane (“PEM”), to provide ion transport between the anode and cathode.
In proton exchange membrane type fuel cells, hydrogen is supplied to the anode as fuel and oxygen is supplied to the cathode as the oxidant. The oxygen can either be in pure form (O2) or air (a mixture of O2 and N2). PEM fuel cells typically have a membrane electrode assembly (“MEA”) in which a solid polymer membrane has an anode catalyst on one face, and a cathode catalyst on the opposite face. The anode and cathode layers of a typical PEM fuel cell are formed of porous conductive materials, such as woven graphite, graphitized sheets, or carbon paper to enable the fuel to disperse over the surface of the membrane facing the fuel supply electrode. Each electrode has finely divided catalyst particles (for example, platinum particles), supported on carbon particles, to promote ionization of hydrogen at the anode and reduction of oxygen at the cathode. Protons flow from the anode through the ionically conductive polymer membrane to the cathode where they combine with oxygen to form water, which is discharged from the cell. The MEA is sandwiched between a pair of porous gas diffusion layers (“GDL”), which in turn are sandwiched between a pair of non-porous, electrically conductive elements or plates. The plates function as current collectors for the anode and the cathode, and contain appropriate channels and openings formed therein for distributing the fuel cell's gaseous reactants over the surface of respective anode and cathode catalysts. In order to produce electricity efficiently, the polymer electrolyte membrane of a PEM fuel cell must be thin, chemically stable, proton transmissive, not electronically conductive and gas impermeable. In typical applications, fuel cells are provided in arrays of many individual fuel cell stacks in order to provide high levels of electrical power.
A significant problem hindering the large-scale implementation of fuel cell technology is the loss of performance during extended operation and automotive cycling. A considerable part of the performance loss of PEM fuel cells is associated with the degradation of the electrocatalyst, probably caused by Pt particle growth, Pt dissolution and carbon corrosion. Carbon has been found to corrode at potentials above 1.2 V and the addition of Pt onto the surface of the carbon increases the corrosion rate considerably at potentials below 1.2 V. These processes lead to a loss in active surface area of the Pt catalyst that leads to loss in oxygen electrode performance. However, cycling experiments reveal that the loss of hydrogen adsorption area alone does not explain the loss in oxygen performance. Additional factors include interference from absorbed OH species, and a possible change in the porosity of the support material affecting voltage loss for oxygen reduction. Therefore, the specific interaction of Pt with the catalyst support can have a significant influence on the stability of performance of the Pt electrocatalyst.
Accordingly, there exists a need for a replacement material for the carbon support to stabilize the oxygen reduction performance of the catalyst.